Anterior lumbar plate and method

ABSTRACT

A plate of biocompatible material is provided having curvature in two planes such that it conforms to the curvature of the L5 vertebral body and to the patient&#39;s lordotic curve. Holes are provided receiving screws for anchorage in the vertebral body and sacrum. A flange or foot portion on the plate provides a wider base end area for support in the L5-S1 interspace. The foot portion is also arranged for appropriate entry angle of screws into the sacrum such as to improve anchorage in the sacrum. Anti-backout and low profile features are incorporated. The anterior lumbar plate is situated to maintain the anterior interbody bone graft in compression by resisting tensile forces during extension.

The present application is a continuation of application Ser. No.09/696,130, filed on Oct. 25, 2000 which is now issued as U.S. Pat. No.6,740,088.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to spinal fixation systems, and moreparticularly to a plate to immobilize the L5 vertebra with respect tothe S1 vertebra.

2. Description of Prior Art

Various types of plating devices and systems, have been used tostabilize portions of the spine. For cases in which interbody fusion isdesired in the lumbar-sacral region, stabilization using plating hasbeen preferred by many surgeons for good fixation and to avoid damage tothe vascular and nervous system components adjacent the anteriorsurfaces of the L5 vertebra. A plating system for stabilization of theL5-S1 junction is disclosed in U.S. Pat. No. 5,127,912 issued Jul. 7,1992 to Ray and Ashman. It is a posterior system. While posteriorfixation systems are often used in anterior/posterior fusions, theanterior surgical approach to the fusion is preferred from severalperspectives. Less blood loss and reduced post-operative pain can beachieved. Also, effective anterior plating could avoid the additionalposterior surgery necessary in the past to provide the additionalstabilization needed to establish a reasonable fusion rate.

U.S. Pat. No. 6,045,552 issued Apr. 4, 2000 to Zucherman and Hsudiscloses a plate for immobilizing the L5 vertebra with respect to theS1 vertebra. Earlier patents and publications are cited in that patent.Also, it is understood that Kostuick and Yuan had modifiedanterio-lateral plates, for example (e.g., the Syracuse I Plate) for useon the anterior lumbar spine. Also, it is understood that earlierliterature reported clinical experience (Humphries and Hawk 1951, 1961)with an anterior lumbar plate manufactured by Austenal Company, NewYork. There remains a need for additional stability to an anteriorlumbar interbody fusion using the same anterior surgical site forplating.

SUMMARY OF THE INVENTION

Described briefly according to the illustrated embodiment of theinvention, a plate is provided having curvature in two planes such thatit conforms to the curvature of the L5 vertebral body and the patient'slordotic curve. Holes are provided receiving screws for anchorage in thevertebral body and sacrum. The screws and receiver holes in an upperportion of the plate are generally perpendicular to that portion of theplate. A lower portion of the plate is formed with a flange or footportion which provides a wider base end area for support on the upperface of S1 in the L5-S1 interspace. The foot portion is also arrangedfor appropriate entry angle of screws into the sacrum such as to improveanchorage in the sacrum. The screws and receiver holes in the lowerportion of the plate are through the front and bottom walls of the lowerportion of the plate and at a steep angle relative to the front of theplate and engaged with the cortical bone of the sacrum at the superiorend plate and at the S1-S2 junction. The foot portion also incorporatesanti-backout and low profile features. The anterior lumbar plate issituated to maintain the anterior interbody bone graft in compression byresisting tensile forces during extension. The plate can also beextended to the L4-L5 junction by increasing the length of the plate andinclusion of holes in the upper portion for anchoring to L4.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an anterior lumbar plate assemblyaccording to a typical embodiment of the present invention.

FIG. 2 is a side elevational view of the plate fixed at the L5-S1junction of the lumbar/sacral region shown schematically.

FIG. 3 is an anterior view of the plate of FIGS. 1 and 2 fixed at theL5-S1 junction.

FIG. 4 is a sectional view taken at line 44 in FIG. 3 and viewed in thedirection of the arrows and showing the plate fixed to L5 and S1, S2,shown schematically and fragmentarily.

FIG. 4A is an enlarged fragment of FIG. 4.

FIG. 5 is a top plan view of the anterior lumbar plate.

FIG. 6 is a bottom plan view of the plate.

FIG. 7 is a schematic illustration of the load bearing of the plate onthe ring apophysis of the sacrum.

FIG. 8 is a fragmentary sectional view through a portion of the plate 11at line 8-8 in FIG. 3 and viewed in the direction of the arrows.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiment illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications in the illustrated device, and such further applicationsof the principles of the invention as illustrated therein beingcontemplated as would normally occur to one skilled in the art to whichthe invention relates.

The anterior lumbar plate 11 according to the illustrated embodiment ofthe invention is generally pear-shaped as viewed from the front in FIG.3 to reduce contact with great vessels, but is curved in a verticalplane as shown in FIGS. 2 and 4, and is curved in a horizontal plane asshown in FIGS. 5 and 6. This facilitates fitting around the vertebra L5and enables the anterior face 11A (FIGS. 2 and 4) to smoothly follow thecurvature of S1 and the spinal column above it. The upper portion 11P ofthe posterior face follows the same curves but changes at the footportion 11S so it can partially enter the intradiscal space between L5and S1. It is preferable that the upper end 11U of the plate be slightlylower than the top surface 12 of L5. The lower face 11B of the footportion 11S of plate 11 is intended to be located at or immediatelyabove the superior plate face 13 of S1.

Depending upon the anatomy of the patient, the overall height betweenthe bottom face 11B and top edge 11U of the plate is likely to bebetween 4.0 centimeters and 5.4 centimeters with an average of about 4.5centimeters. The overall width may be about 2.8 centimeters at thenarrow portion and about 4.0 centimeters at the foot portion. The depth(thickness) between front and rear faces of the upper portion of theplate is about 5 millimeters and increases from slot 26 down to amaximum of about 9 millimeters at the face 11B. As mentioned above, ifit is desired to extend the plate to stabilize two levels, that can besimply done by increasing the overall height between the bottom face 11Bof the plate and the top edge of the plate and adding screw holes of thetype discussed below to anchor into the L4 vertebra.

As shown in FIGS. 3 and 4, there are two screw holes in the upperportion of the plate and which receive screws 16 which are screwed intoL5. The foot portion of the plate has two recesses 17 with lower wallportions 18 curving inwardly FIG. 4A and providing screw guide entrancesto holes 19 which extend from there through the bottom face 11B. Screws20 are received downward through holes 19 and screwed into and throughboth cortical bone at the superior end plate and cancellous bone of S1,and through cortical bone of both vertebral segments at the junctionS1-S2. As an example, the screws may be 6.5 mm diameter cancellous bonescrews. The screw hole axes 19A are also oriented from the screw entrysurfaces of recesses 17 toward the longitudinal axis A of the platewhich is usually substantially co-planar with the mid-plane of thespine. The trajectory for screws 20 and resulting angle B and the angleD (FIG. 4A) of the plane P relative to the plane of the plate bottom11B, are determined by the surgeon using direct visual and X-rayfluoroscopy of the sacrum. It is expected that the included angle (B) islikely to be between 10 and 30 degrees if measured in a plane P of thescrew axes (FIG. 4) or a plane close to the screw axes if they are notco-planar. The underside of the heads of screws 20 are preferablyrounded above the smooth shank portions of the screws received in theholes 19, so that the screw heads can become well seated in the curvedseating surface 18 by permitting some angulation of the screws relativeto the plate 11 as the screws are installed in the sacrum and tightenedin place.

Anti-backout means are provided. In the illustrated example, ananti-backout screw 21 (FIG. 3) is screwed into the plate and extendsover the heads of the two screws 16 to prevent them from backing out ofthe bone L5. Screw 21 may be of the nature shown in U.S. Pat. No.5,364,399 to Lowery et al. Alternatives are possible. One example is aninterlocking wedge feature of anti-backout screw and bone screws toprevent them from backing out. In that connection, and referring to FIG.8, which is a section taken at line 8-8 in FIG. 3 and viewed in thedirection of the arrows, in this instance, the particular screws arebone screws 16A which are anchored in the vertebral body and seat on thescrew shoulders 11S of plate 11 and anchor the plate 11 to the vertebralbody L5. The anti-backout screw 21 is threaded into the plate 11 at theinter-engaging threads 21T. The angled heads of these screws as shown at16H for screws 16A and 21H for the anti-backout screw 21, act as maleand female Morse tapers. The holes in plate 11 are counter-sunk at thediameter of the largest part of the head of the bone screw (i.e. at thebottom of the head). The set screw 21 is placed in the threaded aperturebetween the bone screws in position to mate with the heads of the bonescrews. The countersink bottom surface 11S may be at the same depth forall three screws.

The set screw arrangement can be used regardless of whether the screws16 or 16A are rigid or semi-rigid. If the geometry of the plate 11 issuch that the set screw 21 seats rigidly against the heads of screw 16,it is considered a rigid construct. If, however, a gap is left betweenthe underneath side of the set screw 21, and the top of the bone screws,the bone screws are semi-rigid, allowing motion superiorly andinferiorly, effectively allowing subsidence of the interbody graft. Inthe example of FIG. 8, the set screw 21A is seated on the top of 11S ofthe countersink (counter-bore) in the plate, and allows some gap betweenits Morse tapered face 21H and the matable Morse tapered faces 16H ofthe bone screws. But in the event of any tendency of the bone screw toback out, the wedging between those surfaces will take place andterminate any back-out. This arrangement of the interlocking taperbetween the two bone screws and the anti-backout screw can be expandedin a way such that the anti-backout screw is centered in an array offour bone screws.

Referring now to FIG. 7, there is shown the shape 11B of the bottom ofplate 11 superimposed on the ring apophysis of the sacrum S1,demonstrating the plate 11 load-bearing on the strong ring apophysis forimproved long-term fixation.

Anti-backout screws 22 are provided in the recesses 17 and positioned toprevent the screws 20 from backing out of the S1 bone. Otheranti-backout approaches may be used. Another example is anickel-titanium, shape memory alloy collar arrangement as disclosed inU.S. Pat. No. 5,578,034 to Estes. The near-vertical orientation of thescrews 20 relative to the plate face 11B, and thereby to the superiorend plate surface of S1, enables screw anchorage in dense cortical boneat the sacral promontory and at the S1-S2 junction without risk of thescrews entering the sacral canal SC. In addition, the orientation of theaxes of the two screws 20 at an angle B (FIG. 3) assists in theanti-backout or pull-out feature of this invention as it traps a wedgeof cancellous bone at CS between the two screws 20, and which is veryresistant to being pulled out.

As shown in FIGS. 2 and 4, the bottom plate face 11B of the wedge orfoot portion of the plate 11 is at approximately 90 degrees (angle C)with the anterior face of the plate 11. Depending upon the normal anglebetween the superior end plate of S1 and the inferior end plate surfaceof L5 as well as can be determined for a patient, the end plate angle Cof the plate furnished for the procedure might be selected from 80 to100 degrees.

In addition to appropriately choosing a size of plate to fit thepatient, a plate formation assistance feature is provided. In theillustrated embodiment of the plate, this is a slot 26. This slot isabout 3 millimeters wide, 18 millimeters long and located about midwaybetween the upper end 11U and lower end 11B of the plate. The locationis intended to help the surgeon appropriately bend the plate about anaxis S (FIGS. 2 and 3) perpendicular to the mid-plane and locatedimmediately below the level of the lower surface 14 of L5. Thisfacilitates the bending deemed necessary by the surgeon to best fit theplate 11 to L5 and to S1 with the foot portion 11S extending somewhatinto the anterior portion of the disc space. This is to avoidnecessitating resection of some of the lower anterior portion of L5 toreceive the upper portion 11A of the plate as could occur if the bendwere too low and, if the bend were too high, having the anterior surfaceof the plate at the bend projecting too far out in the anteriordirection with attendant risk of impingement of the vasculaturestructures and sympathetic nerve bundle.

As it is possible to encounter in different patients, a range of spacebetween the superior plate surface 13 of S1 and inferior plate surface14 of L5, anywhere from 12 to 18 or so millimeters at the anterioredges, it can be desirable to provide a selection of several sizes ofplates and locations of the relief slots, to provide the optimum choicesfor the spinal surgeon. As mentioned above, if it is desirable toprovide fusion at more than just the one vertebral space described, theplate can be made taller to cover an additional one or moreintervertebral junctions, with screws such as 16 installed in 14 andsuch higher vertebrae as are associated with the fusion junctions.

An example of material which may be employed in the use of the inventionis a T1 6Al-4V titanium alloy according to Standard ASTM F-136. This maybe associated with various types of interbody fusion device or devicesand bone graft materials 31. Examples are shown and described in U.S.Pat. No. 5,397,364 to Kozak and Boyd. Of course, the present inventionmay be used with other types of materials, surface finishes, andinterbody fusion devices of bone dowel, push-in cage, screw-in cage,with bone graft and/or graft substitute material or other types ofdevices suitable for such fusion applications. The anterior surface isvery smooth with rounded edges to avoid damage to the vascular andnervous systems.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiment has been shown and described and that allchanges and modifications that come within the spirit of the inventionare desired to be protected.

1. An orthopedic implant apparatus with an anti-backout device formaintaining bone screws in the apparatus, the apparatus comprising: aplate including opposite sides defining a profile extending along aheight of said plate between a top end and an opposite bottom end, theplate including a front face and a rear face defining a first depthadjacent the top end and increasing from an intermediate portion of theheight to a second, greater depth adjacent the bottom end to define acaudally-directed bottom surface, said height sized between said top andbottom ends to extend from an upper vertebra to a lower vertebra; atleast one upper hole through said plate adjacent to said top end toreceive a screw for passage into the upper vertebra, wherein said atleast one upper hole opens at the rear face of said plate, said rearface being configured to fit an anterior face of the lower vertebra andincluding a caudally-directed portion of the second, greater depthconfigured to engage bone and/or tissue adjacent the lower vertebra; andat least one lower hole through said plate adjacent to said bottom endto receive a screw for passage into the lower vertebra, wherein saidplate includes a first width between said opposite sides across said atleast one upper hole and said plate includes a second width between saidopposite sides across said at least one lower hole, wherein said firstwidth is less than said second width; wherein said anti-backout deviceincludes a surface configured for engagement with and disposed adjacentto at least said at least one upper hole to prevent the screw in said atleast one upper hole from backing out of the upper vertebra when engagedto the upper vertebra through said at least one upper hole.
 2. Theapparatus of claim 1, further comprising an anti-backout device adjacentto said at least one lower hole to prevent the screw in said at leastone lower hole from backing out of the lower vertebra when engaged tothe lower vertebra through said at least one lower hole.
 3. Theapparatus of claim 1, wherein said front face opposite said rear faceand said opposite sides define generally a pear shape when viewed fromsaid front face of said plate.